The present invention relates to an apparatus for scanning the surface of a specimen such as a semiconductor device two-dimensionally with a focussed, charged beam, and more particularly to an apparatus which can deflect a focussed, charged beam in a wide angular range to scan a large surface area of a specimen with the focussed, charged beam. Although only a scanning electron microscope according to the present invention will be explained later, by way of example, the present invention is not limited to the scanning electron microscope but is applicable to an apparatus, in which a charged particle beam is deflected to scan a specimen surface therewith.
In a conventional scanning electron microscope, as described on page 17 of a Japanese text book entitled "SOSAKENBIKYO NO KISO TO OYO (Elementary Knowledge and Applications of Scanning Electron Microscope)" published by KYORITSU SYUPPAN Co., Ltd. on Dec. 1, 1983, an electron beam is arranged so as to pass through the center of an objective lens, and a specimen irradiated with the electron beam is moved horizontally so that the electron beam impinges upon a desired surface portion of the specimen, in spite of the fact that the electron beam can be deflected by an electric or magnetic field so as to cover the desired surface portion of the specimen. This is because, in order for the scanning electron microscope to realize a spatial resolution of about 100.ANG., the electron beam is required to pass through the center of the objective lens, to minimize the aberration with respect to the objective lens.
FIG. 11 is a schematic diagram showing the deflection of electron beam in the conventional scanning electron microscope. Referring to FIG. 11, in order to prevent the spherical aberration of an objective lens 3, an electron beam 5 is deflected by deflectors 1 and 2 so that the beam 5 always passes through the center of the lens 3. When the deflection angle at the center of the lens 3 is expressed by .beta., the diameter .DELTA.d of disk of confusion is given by the following approximate equation: ##EQU1## where V indicates an accelerating voltage, .DELTA.V the energy spread of the electron beam, IN the ampere-turn of the objective lens, and L the distance between the objective lens and a specimen 4. For example, in a case where the accelerating voltage V is 10KV, the energy spread .DELTA.V is 1V, a ratio IN.sqroot.V is 15, the distance L is 50 mm, and the deflection angle .beta. is 0.1 radian, the moving distance of the electron beam at the surface of the specimen will be 5 mm, and the diameter of disk of confusion will be 1.mu.. Thus, the deflection angle .beta. has been made less than 10.sup.-3 radian, to make .DELTA.d smaller than 100.ANG.. Accordingly, in order to observe a large surface area of the specimen 4, it is necessary to move the specimen horizontally.
In recent years, a scanning electron microscope has been used as an electron beam tester for observing the internal operation of an integrated circuit. In an electron beam tester, a completed large scale integration circuit is used as a specimen, and the operating state thereof is observed. Accordingly, there arise problems that a large number of lead wires connected to a specimen have to be introduced in a vacuum chamber and the specimen has to be horizontally moved within the vacuum chamber. In short, the electron beam tester has a drawback that such a specimen has to be horizontally moved in vacuum.